1. Technical Field
The present disclosure relates to a method for controlling a three phase equivalent voltage of a multilevel inverter, and more particularly, to a voltage control method capable of providing a continuous operation by performing a three phase equivalent voltage control using an offset voltage (or a zero sequence voltage) without bypassing power cells operating normally to maintain an equivalence of a three phase line-to-line output voltage when some of power cells of a cascaded H-bridge (CHB) multilevel inverter are failed.
2. Description of the Related Art
A multilevel inverter has been mainly studied in a field to which a medium voltage is applied, especially, in a field of an electric motor drive.
There is difficulty in applying a voltage source inverter to a variable drive of a large-scale alternating current electric motor requiring over a medium voltage due to limitation to a rated voltage and voltage stress of a switching device that configures an inverter.
A voltage source inverter requiring over medium voltage may be exemplified as a 2-level inverter that has characteristics of equivalent voltage stresses as many as the number of connected switching devices using a serial connection of the switching devices. However, when an electric motor is driven using such a 2-level inverter, it may be difficult to actually apply the 2-level inverter to an industrial field because there are problems such as degradation, insulation and the like of an electric motor caused by a high dv/dt, a voltage reflection, high total harmonic distortion (THD), high switching loss, and the like.
In order to address such problems, a variety kind of multilevel inverters have been being developed, and a cascaded H-bridge (CHB) multilevel inverter among such inverters shows most prominent characteristics in aspects of input and output voltages and current quality. Each phase of the CHB multilevel inverter is implemented through a serial connection of power cells, each of which is configured with a single H-bridge inverter.
The CHB multilevel inverter has advantages including modularization ability, high reliability, operation continuity, low total harmonic distortion (THD) of an input current, and the like. And, by virtue of such advantages, the commercialization of the CHB multilevel inverter has been actively made such that an inverter product for driving a medium voltage electric motor is currently being released by numerous manufacturers.
A power cell of the CHB multilevel inverter is made of a three phase rectifying circuit, a direct current (DC) link capacitor, and an H-bridge circuit, and N power cells are connected in series to form a single phase. When k power cells are failed among the N power cells, the CHB multilevel inverter bypasses the k power cells to enable a continuous operation. Meanwhile, when the k power cells are bypassed at a single phase, non-equivalence occurs in each of line-to-line voltages. In order to address such non-equivalence, the k power cells should be also bypassed at other phases. In such a case, redundancy that the CHB multilevel inverter has inherently is decreased, and also an output voltage is decreased by an amount of (N−k)/N.